How To Bake A Tape
A lot of tape manufactured in the mid-to-late 1970's is starting to
come out of storage now for remixing and re-issue, and engineers are
finding that it won't play. The surface of the tape has become gummy
and it sticks to the heads and fixed guides of the tape transport,
squealing, jerking, and, in extreme cases, slowing down or stopping
the tape transport. This problem has cropped up on all brands of
tape, but is nearly always fixable, at least temporarily.
Tapes can exhibit two different problems as a result of long term
storage; binder breakdown and lubricant breakdown. Lubricant
breakdown, which is fairly rare, leaves a white residue when the tape
is run over the heads. Binder breakdown, the more common failure
mode, leaves a dark, gummy residue, and is fixable by gentle heating
("baking") of the tape. Fixing lubricant breakdown requires careful
cleaning of the tape and possibly applying fresh lubricant. Baking
will not solve the lubricant breakdown problem and may make it worse.
Make sure you know which problem you have before you put a tape in the
oven.
Here's where the stickiness comes from. The binder is the chemical
compound that holds the oxide particles together and sticks them to
the tape backing. Under humid conditions (which means anything but
controlled low-humidity storage), the polyurethane used in the binder
has a tendency to absorb water. The water reacts with the urethane
molecules, causing them to migrate to the surface of the tape where
they gum up the tape path during playback.
Short strings of urethane molecules are particulary prone to water
absorption, while long strings make the coating mixture too viscous to
produce good tape. Middle-lengh strings are the best, but the tape
manufacturers didn't know this at the time, and didn't always know
what they were getting. In the case of Ampex tape, tapes most likely
at risk are 406 and 456 manufactured from approximately 1975 through
1984. During those years, Ampex tested the goop they got from their
binder suppliers simply by measuring viscosity. Unfortunately, the
long and short strings average out, viscosity-wise, to a viscosity
about the same as the ideal medium strings, so some tape was
inevitibely manufactured with an overly great proportion of short
urethane strings in the binder. In the worst cases, as little as 3
days' exposure to 70% relative humidity can cause a tape to become
gummy, but typically, it takes 2 to 15 years under normal,
people-friendly ambient conditions. In 1984, Ampex started doing it's
incoming inspection with a high pressure gas chromatograph (that's
when it was invented), and was able to more accurately determine the
molecular makeup of it's binder, and control production much more
carefully. Better things for better living through chemistry.
The good news is that the "sticky shed syndrome" resulting from water
absorption by the short urethane molecule chains is almost always
fixable. The process for repair is commonly know as "baking a tape".
The fix lasts about a month under normal storage conditions, and Ampex
claims that a tape can be re-baked any number of times without ill
effects. Best advice, though, is to make a copy of the tape on first
playing, and work with the copy.
To bake a tape, you want to expose it to even heat, ideally at 130
degrees F, with a variation of less than plus or minus 10 degrees.
Too cool and the process is ineffective, too hot and you're starting
to risk increasing print-through.
There are several kinds of ovens you can use. One thing you DON'T
want to do is stick it in your kitchen oven and turn the heat on
"low". Most oven thermostats don't go low enough, don't provide good
enough temperature control, and a gas flame generates quite a bit of
water vapor, exactly what you're trying to get rid of.
It's important that the tape be packed smoothly before baking.
Chances are it will be if it's been cared for as a master tape should,
but if it needs to be re-packed, this should be done by winding the
tape at play speed on to another reel using a tape deck on which the
heads can be removed, and with the tape threaded so that it doesn't
pass over any fixed (non-rotating) guides.
The hair-dryer-in-a-cardboard-box, a method attributed to George Horn
of Fantasy Records, is a simple, inelegant home made tape baking oven
that works well and can be made from things you're likely to have
around the house.
Bill of Materials:
A cardboard box about 14" on a side (a tape carton is about right, but
a wine carton will do) with the flaps cut off
A 1500 watt electric hair (blow) dryer
A wire shelf (I borrow one from the kitchen oven)
A couple of 2x4's to support the wire shelf
A meat thermometer, the kind with the round dial and pointed probe
intended for sticking into roasts. If you have to buy one, I
recommend the ones that Starbucks sells for measuring the
temperature of hot milk for cafe latte. You can get a digital
one if you want, but it's not required since you're baking
analog tapes. (you didn't expect me to get through this
without at least a little tongue-in-cheek did you?)
Something to space the tape reels in a stack if you're baking several.
The cores from a 10-1/2" tape reels are good.
The wire shelf is raised about 3" off a table top on the 2x4's, the
tape reels stacked on the shelf, and the box placed so it covers the
reels. Cut a hole in one side of the box in the center near the top,
and insert the nozzle of the hair dryer. I found that I needed to
hold the handle of my hair dryer in a vise in order to keep the whole
rig from falling over. This is the only part that requires some
in-place engineering and creativity. Stick the meat thermometer
through the opposite side of the box somewhere near the middle, so
it's not directly in the blast of the hot air. Of course the tape is
removed from it's box, but it is not necessary to remove it from the
reel.
Turn on the dryer and keep a close watch on the thermometer. George
Horn recommends low heat range and low fan speed. I found that with
my dryer, I could never get above about 100 degrees like that and it
required the high heat range and high fan speed to make it up to 130
degrees. Your mileage will vary. Remember, there's no thermostatic
control, so be sure to check the temperature at least every half hour
to make sure things aren't running away, at least until you get to
know your hair dryer. The make and model of the hair dryer, the size
of the box, and the distance the rack is spaced off the table top all
affect the temperature. It's best to experiment first without your
prized master tape until you find a combination that yields a steady
130 degrees after the initial warm-up.
+------------------------+
| |
/--------------| | ------Box
Blow / _ _______| |
Dryer / / | |
| __/ | |||||||||||||||| ----------Tape Reels
| | | XXXXXXXX |
|__| | XXXXXXXX ======O ----Thermometer
| |||||||||||||||| |
| XXXXXXXX -------------Spacer
| XXXXXXXX |
=========================================== ----Wire shelf
| | | | -----2x4
| | | |
```````````````````````````````````````````````````````````````````
Table Top
Baking time ranges from about 4 hours for 1/4" tape to 8 hours for 2"
tape. It's not critical. You can't over-bake unless you leave it for
a day or so (and your hair dryer will probalby burn out before then),
and if you under-bake and the tape is still gummy, you can bake it
more. After you shut off the heat, leave the tape to cool down to
room temperature before running it through the deck again.
If you want a more elegant solution, check your local appliance shop
for a Faberware (or equivalent) convection oven, but make sure it's
large enough to accommodate the size tape reels you use. These run
about $150 and might be a worth while investment if you have a large
amount of tape to bake. Hair dryers aren't designed for continuous
duty and you'll probably go through a few if you have a large project.
Microwave ovens are totally inappropriate for the job, though I've
read at least one report of success with one.
Lately, I've been doing tapes in my pilotless kitchen oven by
replacing the 25 watt appliance lamp inside with a standard 100 watt
light bulb, and putting in a muffin fan salvaged from a dead PC power
supply to circulate the air. That gives me a nice stable 130 degrees,
but it took some experimenting with the fan speed and placement.
Without the fan, it never got beyond about 110 degress, and I found
that a 150 watt bulb (my first test) wouldn't allow me to get below
140 degrees. The friend who told me about this approach says he does
it in his kitchen oven with the standard bulb that was in there when
he bought the house. Go figure.
The technical jargon about the molecules comes from an article by
Philip De Lancie in the May 1990 issue of Mix Magazine, where he
quoted sources from Ampex. I'm no molecular chemist, just giving
credit where it's due (and relieving myself of the responsibility for
their errors).
--
I'm really Mike Rivers (mrivers@d-and-d.com)
P.S. It is really important to let the tape cool down SLOWLY to room
temperature, or the stickies will come right back. Turn off the heat,
leave the oven/box closed, come back in 3 or 4 hours. When it's
completely cool, then you can play it again.
For further information on this topic, read about Wendy Carlos'
experiences at
www.wendycarlos.com/news.html which also contains a link to a
copy of Eddie Ciletti's article in EQ, which lists recommendations for
using a readily available food dehydrator instead of a lab oven.